1. Field of the Invention
The invention relates to a method and a device for compensating the polarization mode dispersion (PMD) of a transmitted optical signal by means of a polarization controller coupled to a differential group delay generator, whereby the polarization controller is controlled by a feedback signal computed by a control algorithm.
2. State of the Art
A method and a device of this type are known in the art through EP-0 853 395.
All types of fiber links present the phenomenon of polarization mode dispersion, i.e. the impulse or signal emitted by an emitter terminal and transmitted via a fiber link is received in a deformed state by a receiver terminal. It shows a longer duration than its original duration. This deformation is due to the fact that the optical signal depolarizes during transmission. The signal received at the other end of the fiber link can be considered as two perpendicular components, where one corresponds to a state of polarization (SOP) with maximum propagation velocity and the other corresponds to an SOP with minimum propagation velocity. In other words, the signal received at the other end of the fiber link can be considered as being constituted by a first signal polarized with a privileged SOP arriving first and a second signal propagating with a second delayed SOP arriving with a delay called differential group delay, which depends in particular on the length of the fiber link.
If the emitter terminal emits an optical signal with a very short impulse, the optical signal received by the receiver terminal consists of two successive impulses polarized perpendicular to one another and having a time delay corresponding to the differential group delay. This delay can be in the order of 20 picoseconds for a link of 100 kilometers comprising a single mode fiber produced a few years ago. The deformation of the impulses received by the receiver terminal can cause errors in decoding the transmitted data, hence the pollarization mode dispersion is a limiting factor for the performance of optical links, analog as well as digital.
Fibers with strong polarization mode dispersion, also called polarization maintaining fibers, are known, which allow supplying a fixed differential group delay by using short sections of fiber. Optical compensation of the polarization mode dispersion can be realized by disposing such a component, or a whole differential group delay generator, between two perpendicular polarization modes which result from a fiber link with strong polarization mode dispersion. This can be implemented either by using a polarization maintaining fibre (PMF) with the same differential group delay as the fiber link, but by exchanging the slow and fast principle SOPs, or by forcing a principle SOP of the system consisting of the fiber link and the PMF to coincide with the SOP of the source of emission. In order to do this, a polarization controller is placed between the fiber link and the PMF.
The value of the differential group delay and the principle SOPs of a link vary over time with temperature and vibrations. A compensation means must therefore be adaptive and the differential group delay of the PMF must be chosen such that it is at least equal to all the values of differential delay that are to be compensated.
Such a means for compensating the PMD in a system of optical transmission became known by the EP 0853 395 A1. The means comprises a polarization controller and a differential group delay generator . The controller and the generator are disposed between the fiber link and the receiver terminal. A feedback loop measures the degree of polarization (DOP) of the optical signal delivered by the DGD generator and feeds the polarization controller such that the DOP measured is optimized. However, using the DOP as feedback variable requires a complicated algorithm for computing the control of the polarization controller. Furthermore, the rewinding of birefringent elements in the polarization controller constitutes a major problem.
The object of the present invention is therefore to provide a method and a device for a faster and more accurate control of the polarization controller.
In a first aspect of the invention, the problem is solved by a method and a device for compensating the polarization mode dispersion (PMD) of a transmitted optical signal by means of a polarization controller coupled to a differential group delay (DGD) generator, whereby the polarization controller is controlled by a feedback loop, said feedback loop implementing an optimisation algorithm to optimize a feedback parameter of the output signal of the DGD generator. The algorithm takes into account the state of polarization (SOP) of an optical signal determined from the output signal of the polarization controller or from the output signal of the DGD generator. The feedback parameter can be the DOP, or a measure of the electrical spectral width, or a measure of the eye opening, etc. Preferably it is the DOP, as the measurement procedure is common. If the feedback parameter used is the DOP, it is more practical to measure it at the output of the DGD generator (since the same equipment is used), but the theory holds when it is measured at the input of it.
In a variant of the inventive method the SOP is computed from the Stokes parameters measured from the output signal of the DGD generator. For the feedback loop a very high speed is required. The Stokes parameters can be easily measured and from the measured values the position of the SOP on the Poincarxc3xa9 sphere can be easily computed. Hence the measurement of the Stokes parameters is particularly suitable for a fast and accurate feedback to the polarization controller. The control signals computed by the algorithm are fed back to the polarization controller. Measurement of the Stokes parameters allows to make the algorithm easier and faster. Thus the processing of high bit rates becomes possible in single mode fiber links.
In a preferred variant of the method the polarization controller comprises at least one birefringent element and for each birefringent element an angle xcex1 is fixed and a rotation angle xcex2 is variable, where the angle xcex1 is the angle between a rotation axis on the equatorial plane of the Poincarxc3xa9 sphere and the x-axis of a coordinate system and the rotation angle xcex2 defines the rotation around the rotation axis. A birefringent element changes the SOP at its input into another SOP at its output by a rotation of the SOP on the Poincarxc3xa9 sphere.
A polarization controller consists of a cascade of birefringent elements with either a variable xcex1 or a variable xcex2, or both. If a birefringent element is to allow a variable xcex2, the birefringent element must be wound back once the maximum xcex2 is reached since it cannot reach infinite values and other birefringent elements would have to compensate for this element. The rewinding process requires a complicated algorithm. In contrast, if the angle xcex1 is variable and xcex2 is fixed, the complicated rewinding process of the birefringent elements can be facilitated or even avoided.
Advantageously, the impact of each birefringent element on the position of the SOP on the Poincarxc3xa9 sphere is determined. If the position or the variation of the SOP on the Poincarxc3xa9 sphere is known, the algorithm in the feedback loop can take this into account and vary its step accordingly. The algorithm takes a larger step if the SOP is close to the rotation axis and a smaller step, when the SOP is further away from the rotation axis. In addition, knowing the variation of the SOP due to each birefringent element in the polarization controller allows to combine the birefringent elements such that the voltages applied to them and thus their position can be changed without changing the SOP. This increases the speed of the rewinding process. Determining the SOP and using it in the algorithm in the feedback loop could also be used with polarization controllers of the state of the art, i.e. where xcex2 is fixed and xcex1 is variable.
In a preferred embodiment of the inventive device, the polarization controller comprises at least one birefringent element. The control signals determined by the algorithm in the feedback loop are fed back to the polarization controller and the positions of the birefringent elements in the polarization controller are adjusted in order to compensate for PMD by maximizing the DOP. The constraints on the birefringent elements can be relaxed by use of the inventive device since the algorithm required to compensate for imperfections of the birefringent elements becomes less complicated.
Preferably, the DGD generator comprises at least one polarization maintaining fibre (PMF). The polarization maintaining fiber compensates for the differential group delay resulting from the fiber link by launching the fast signal part into the slow eigenstate of a polarization maintaining high birefringent fiber and vice versa.